Immiscible injection of solvent in dilution chilling of waxy oils

ABSTRACT

A dewaxing process is described comprising contacting a waxy lubricating oil with a dewaxing solvent in a chilling zone divided into a plurality of stages. The dewaxing solvent is injected into the chilling zone under conditions such that the solvent and oil are immiscible at the point of injection. At least a portion of the wax is precipitated from the oil and is separated therefrom under conditions such that the solvent and oil are miscible. Solvent injection is at a temperature preferably below 0*F. when the solvent comprises 40 volume percent methylethylketone and 60 volume percent methylisobutylketone.

United States Patent [191 Eagen et al.

[451 Dec. 18,1973

[54] IMMISCIBLE INJECTION OF SOLVENT IN 3,681,230 8/1972 Eagen et al. 208/33 DILUTION CHILLING WAXY OILS 2,760,904 8/1956 Ford 208/33 3,239,446 3/1966 Demeester et a1 208/33 [75] Inventors: John F. Eagen; David A. Gudehs,

both of Sarma Ontano Canada Primary Examinerl-lerbert Levine [73] Assignee: Esso Research and Engineering Attorney-Leon Chasan et al.

Company, Linden, NJ.

[22] Filed: Mar. 13, 1972 57 RA T PP .2 234,405 A dewaxing process is described comprising contact Related US Application Data ing a waxy lubricating oil with a dewaxing solvent in a chilling zone divided into a plurality of stages. The de- [63] fgg gts 'xg of waxing solvent is injected into the chilling zone under conditions such that the solvent and oil are immiscible [52] U S 208/33 208/37 at the point of injection. At least a portion of the wax 51] 6 g 43/08 is precipitated from the oil and is separated therefrom 58] Fieid 208/33 under conditions such that the solvent and oil are miscible. Solvent injection is at a temperature preferably 56] References Cned below 0F. when the solvent comprises 40 volume percent methylethylketone and 60 volume percent me- UNITED STATES PATENTS thylisobutylketona 3,105,809 /1963 Butler et al. 208/31 3,642,609 2/1972 Mayer et a1 208/33 17 Claims, 3 Drawing Figures LIQUID-LIQUID MISCIBILITY OF MEK-M1BK WILL VARY FEEDS 80 I I 1 1 TYPICAL VISCOSITY sus AT IOOF 60 BOON =9eo BAROSA 5s= 410 150 N =|5o 900 N U. BAROSA 56 D: E E 2 1". t o g E 8 g 2o l 1 I I 0 10 2o 30 4o LWscICIEK so so 100 90 a0 70 so 50 4o 30 20 1o 0 PMFNTED DH? 18 0975 FIGURE 2 D.A. GUDELIS PAIENIEUuu: 1 8 I975 31791894 sum 3 CF 3 FIGURE 3 EFFECT OF SOLVENT TEMPERATURE ON PERFORMANCE FEED BATON ROUGE BAROSA 56 SOLVENTZ 40/60 MEK /M|BK SOLVENT/FEED DILUTION RATIOS 2.8/l

I I I SOLVENT/OIL BECOMES IMMISCIBLE AT +7 F SLACK WAX OIL CONTENT, WT. /o(SBA) 20 IO 0 -|o -20 SOLVENT INJECTION TEMPERATURE F INVENTORS J.F. EAGEN D. A. GUDELIS IMMISCIBLE INJECTION OF SOLVENT IN DILUTION CI-IILLING OF WAXY OILS CROSS-REFERENCE TO RELATED APPLICATIONS This a continuation-impart of copending application Ser. No. 60,252 filed Aug. 3, 1970 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wax-oil separation process. More particularly, the invention relates to a process for dewaxing a wax-containing lubricating oil stock. Still more particularly, the invention relates to a dewaxing process comprising contacting a waxy lubricating oil with a dewaxing solvent thereby precipitating at least a portion of the wax from the oil. Still more particularly, the subject process comprises contacting a waxy lubricating oil with a dewaxing solvent under conditions such that the oil and solvent are immiscible, precipitating at least a portion of the wax from the oil and separating the wax therefrom under conditions such that the oil and solvent are miscible.

2. Description of the Prior Art It is known in the prior art to dewax petroleum oil stocks by cooling an oil/solvent solution in a scraped surface exchanger. In this type process, the oil and selective solvent are admixed at a temperature sufficient to effect thorough solution of the oil in the solvent. The extent of dilution is dependent upon the particular oil and the particular solvent employed and is adjusted to facilitate easy handling and optimum filtration rates. The solution is cooled at a uniformly slow cooling rate; e.g. 5F./min., under conditions which are controlled so as to avoid any substantial agitation of the so lution during precipitation of the wax. Notwithstanding the carefully controlled conditions used in this type process, there are deficiencies which hamper successful commercial operation. Most significant among these deficiencies is the loss of good heat transfer due to wax deposition on the exchange surfaces. Such fouling has been repeatedly noted after short periods of operation; e.g. 24-48 hours. Associated directly with the loss of good heat transfer is the loss of careful control over the cooling rate and a corresponding loss of uniform crystal growth. This non-uniform crystal growth then results in lower filtration rates. The high pressure drop through the chilling section also reduces the maximum feed rate attainable.

It is also known in the prior art to dewax lubricating oils with hydrocarbon solvents in scraped surface exchangers using the incremental dilution process. In this process the waxy lubricating oil is cooled to well below the cloud point before adding the first increment of solvent to precipitate the wax. The amount of solvent required depends on the nature of the solvent and the viscosity of the lubricating oil. The deficiencies which hamper successful commercial operations in this process are similar to those already mentioned.

'- SUMMARY OF THE INVENTION The present invention relates to a wax-oil separation process comprising introducing a wax-oil mixture into a chilling zone and contacting the mixture-therein with a solvent. The solvent is introduced into the chilling zone at conditions such thatthe solvent is substantially immiscible with the wax-oil mixture. Thereafter, conditions are maintained in the chilling zone such that the solvent and wax-oil mixture are substantially miscible. The wax-oil mixture is cooled and agitated, thereby precipitating at least a portion of the wax from the waxoil mixture, and a miscible oil-solvent mixture containing precipitated wax is withdrawn from the chilling zone. The precipitated wax is separated from the mixture at a temperature at which the oil and solvent are miscible. By operating in this matter, the oil content of the wax recovered from the process, i.e., the slack wax, is reduced.

In one embodiment of the invention, the chilling zone is divided into a plurality of stages and solvent is introduced into at least a portion of the stages. It is important in this invention that the temperature and solvent composition be chosen at each point of solvent injection so that the oil and solvent are substantially immiscible at the injection point. As the solvent diffuses into the oil, it reaches a temperature at which it becomes miscible with the oil. It is also important in one embodiment of the invention that the degree of agitation within at least a portion of the stages in the chilling zone be sufficient to permit substantially instantaneous mixing of the oil and the solvent, i.e., within about a second or less. The high degree of agitation offsets the deleterious effects of shock chilling and permits control of the cooling rate.

The temperature of the solvent and its rate of addition to the chilling zone are controlled so as to permit a desired cooling rate of the oil. The wax which precipitates during the cooling stage may be separated from the oil-solvent mixture by conventional means known in the art. Following separation of the wax, the oil which has a reduced pour and cloud point is recovered from the solution by means well known in the art.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing the relationship between viscosity and miscibility of various oil-solvent mixtures. FIG. 2 is a flow diagram of the dewaxing process.

FIG. 3 is a graph showing the effect of solvent injection temperature on oil content of slack wax.

Referring to FIG. 2 in detail, the oil stock to be dewaxed is conducted from the storage tank 1, through line 2, to the top of the vertical cooling tower 3, where it enters the first stage of the cooler 4a. In this regard it is noted that the shape of the cooling zone is not critical to the invention. However, preferably, the cooling zone will comprise a vertical tower similar to that shown in FIG. 2. The selected solvent is passed from storage tank 5 through line 6, through heat exchangers 7 and 8, where the solvent temperature is reduced to a level sufiicient (I) to permit substantially immiscible injection conditions and (2) to cool the oil to the desired dewaxing temperature. Coolant enters the heat exchangers 7 and 8 through lines 24 and 25, respectively, and leaves through lines 26 and 27. The solvent leaves the heat exchanger 8, through line 9, and enters manifold 10.

The manifold comprises a series of parallel lines providing solvent inlets 1111 to the several stages of the cooling tower 3. The rate of flow through each inlet is regulated by flow control means (not shown). The rate of solvent flow is controlled so as to maintain a desired "temperature'gradient along the height of the cooling tower 3.

The first portion or increment of the solvent enters the first stage, 4a, of the cooling tower 3 at a temperature and composition such that it is substantially immiscible with the oil and where it is substantially instantaneously admixed with the oil due to the action of the agitator 12a. The agitator is driven by a variable speed motor 13 and the degree of agitation is controlled by variation of the motor speed, with due allowance for the flow rate through the cooling tower. It is noted that while mechanical rotary type agitation is shown, any mixing system can be employed in the process so long as it provides the high degree of mixing efficiency required for successful operation of the process. Additionally, it is noted that high levels of agitation are not necessarily required in each of the stages of the cooling tower. In some instances it may be preferred to maintain high levels of agitation in only a portion of the stages, e.g. in the upper stages.

The oil-solvent mixture may pass upwardly or downwardly through the cooling tower 3 (downward flow only has been shown). At various heights along the cooling tower, additional prechilled solvent is introduced to each of the several stages 4, through inlets 11 so as to maintain a substantially constant temperature drop per stage and at the same time to provide the desired degree of dilution. In some instances it may be desirable to add solvent to the tower at only a portion of the injection points. While any number of stages up to about fifty may be employed, in one embodiment, at least six stages should be used. However, it may be desirable, in some instances, to use a laser number of stages.

The oil-solvent solution with precipitated wax passes from the final stage of the cooling tower through line 14 to means for separating the wax from said solution 15. Any suitable means such as filtration or centrifugation for such separation may be employed. The waxsolvent mixture is removed from the separation means through line 16. The solvent is recovered in a suitable separating system 19, which is preferably distillation, through line 17 and the wax exits through line 18. The oil-solvent solution leaves the wax separation means through line 20 and passes to means for separating the oil from solution 21. Any suitable means for this separation may be used, such as distillation or selective adsorption. The oil is removed from the separator and is recovered through line 22. The solvent is removed through line 23. The solvent may be recycled directly or scrubbed to remove impurities before reuse.

To assure immiscible conditions at the injection point in each of the stages of the tower, the following alternative means can also be used, or would be evident to those skilled in the art.

1. The solvent injected into the tower can be split from exchangers 7 and 8 so that warmer solvent from exchanger 7 is injected into the top stages where the temperature inside the tower is higher vis-a-vis the temperature in the lower stages, and colder solvent from exchanger 8 is injected into the lower stages to obtain the required overall dilution ratio and lower exit temperature. For example, the temperature of the solvent to the top stages could be 0F. while the temperature of the solvent to the last six stages could be -F.

2. The immiscible conditions could also be provided by changing the solvent composition being injected into the tower, the solvent still having a final composition such that it is .miscible'with the oil at the-filtering temperature. For example, if 40/60 MEK/MIBK (by volume) is required at the filtering temperature, an 80/20 mixture (by volume) could be injected into the top of the tower and say a 20/80 mixture (by volume) into the bottom of the tower. Since the composition affects the miscibility temperature as shown in FIG. 1, immiscible conditions could be obtained by altering the solvent composition as well as the solvent temperature.

All lubricating oil fractions are suitable for use in this invention. They may be more particularly described as follows:

I. Lubricating Oil Distillates Boiling Range vow-1.0

Gravity 2038 APl SUS at 100F. 504.400

Pour Point 2. Bright Stocks Boiling Range i,0s0F.+

Suitable solvents that may be used in the process include ketones having three to six carbon atoms per molecule such as methylethylketone (MEK), methylbutylketone, and methyliso-butyketone (MIBK) and mixtures thereof as well as mixtures of ketones with olefins, e.g., propylene and acetone, and mixtures of hydrocarbons with ketones, e.g., MEK-toluene which are immiscible at some temperature below the wax separation temperature and become miscible as the temperature rises.

During the operation of the process of the present invention, the petroleum oil stock is fed to the cooling tower 3 at a temperature which can be either above or below its pour and cloud point. In the case of an oil fraction containing a relatively low amount of wax, the oil may be fed at storage temperature. In the case of an oil containing a relatively large amount of wax an elevated temperature will generally be used. In general, the wax content of the oil feed will vary between 10 and 25 wt. percent and the pour and cloud points will range between 70 and 170F. and and l75F., respectively. In general, a feed rate between and 600 bbL/hour will be used; however, it will be apparent to those skilled in the art that higher or lower feed rates can be used.

The solvent, or solvent mixture, will be prechilled to a temperature sufficient to permit cooling of the oil to the desired temperature at which the solvent will be immiscible with the oil. It will be apparent to those skilled in the art that the exact temperature employed will depend upon the amount of oil to be cooled and the amount of solvent to be added to the oil; i.e., the degree of dilution which is sought during the filtration step. The prechilled solvent is added incrementally along the height of the cooling tower so as to maintain a chilling rate below about l0F./minute, preferably below about 8F./min., and most preferably between about 1 and about 5F./min. In general, the amount of solvent added will be sufficient to provide a liquid/solid weight ratio between I] I and 30/1 at the dewaxing temperature and a solvent/oil volume ratio between 1.5/1 and 5/].

In some cases it may be desirable to carry out the chilling in the chilling zone only part of the way and to finish up the chilling in conventional cooling equipment such as scraped surface exchangers. In this case the temperature of the oil-solvent-wax mixture leaving the tower is preferably no higher than about 35F. above the wax separation temperature. It is noted that this combination is also suitable for situations where the above temperature difference cannot be maintained, as in dewaxing to very low temperatures, e.g. 80F. In this instance the temperature differential may be as great as 60F.

In general, the degree of agitation required in this invention can be achieved when the modified Reynolds Number, (Perry, Chemical Engineers Handbook, 3rd, p.l,224, McGraw-Hill, New York, 1959), N e, which is defined by the equation:

Where L agitator diameter, ft.

l= liquid density, lb./ft.

n agitator speed, r.p.s.

LL liquid viscosity, lb.ft.sec. is between about 200 and about 150,000 and the dimensionless ratio of cooling tower diameter to agitator diameter is between about /1 and about 10/1. A turbine type agitator is preferred; however, other types of agitators such as propeller and disc may be used.

The cooling tower may or may not be baffled, but a baffled tower is preferred. The tower is divided into several cooling stages by horizontal circular plates which restrict flow between the stages to an opening in the center of the tower. In general, the dimensionless ratio of the diameter of the restricted flow opening to the diameter of the tower will be between about H3 and about l/100.

In general, the cooling tower will be operated at a pressure sufficient to prevent flashing of the solvent. Atmospheric pressure is sufficient when the ketones are employed as solvents.

PREFERRED EMBODIMENT The invention will be more apparent from the preferred embodiment and working examples set forth below. Turning again to FIG. 2, a lubricating oil distillate fraction, having a boiling range between 700 and 1,300F., a wax content of from about 8 to about 35 wt. percent, and initial pour and cloud points between about 85 and 175F. and 90 and 180F. respectively, is fed from the storage tank 1 through line 2 to the first stage 4a of the cooling tower 3. A solvent consisting of a mixture of methylethylketone and methylisobutylketone is passed from storage tank 5 through line 6 to the heat exchangers 7 and 8, whereby the solvent is cooled by conventional means to a temperature between 0 and -25F. In the preferred embodiment cold filtrate from the separator 21 is used to precool the solvent in heat exchanger 7. The filtrate enters through line 24 and leaves through line 26. The solvent is further cooled in heat exchanger 8 with liquid propane or other suitable coolant, entering through line 25 and leaving through line 27.

According to the invention the solvent is prechilled to a temperature below that at which it is miscible with the oil and which will permit cooling of the oil to a temperature between about 0F. and 40F, although lower temperatures can be used. The cooled solvent passes from the heat exchanger 8 through'line'9 to the manifold 10. In the preferred embodiment the cooling tower is divided into-sixteenstages, .4, and the manifold consists of sixteen parallel solvent inlets R2. The flow rates are adjusted to give the desired temperature drop per stage. Each of the sixteen separate stages is provided with an agitator turbine 12 which is turned at a sufficiently high RPM to produce adequate mixing. The oil and solvent entering the first stage 4a are substantially instantaneously mixed. As the oil-solvent mixture passes downwardly through the cooling tower, it is substantially instantaneously mixed with additional solvent which is added to each stage.

The oil-solvent mixture leaves the bottom of the cooling tower as a miscible mixture of oil and solvent containing precipitated wax. The mixture passes through line M to a separating means 15 which is preferably a rotary vacuum filter. The wax-solvent is passed through line To to a liquid-liquid separating means 19 which is preferably distillation. The wax is recovered through line 18 and the ketone solvent is recovered through line 20 to a liquid-liquid separating means which is preferably distillation. The dewaxed oil is recovered through line 22 and the mixed ketone solvent is recovered through line 23. In one embodiment, the recovered solvent can be recycled for use as dewaxing solvent in the tower. The dewaxed oil which now has a reduced pour point and cloud point may be used directly or subjected to further finishing steps. The miscible oil-solvent mixture containing precipitated wax may also be further chilled as it exits from the tower 3 through line 14 in conventional scraped surface exchangers prior to the filtration step.

EXAMPLE Using the preferred embodiment described above, a heavy neutral lubricating oil having the following specifications:

Bpiling Range 825-1050F.

Dry Wax Content 20 percent Pour Point 130 Cloud Point 132 was dewaxed according to the process of this invention. The oil fraction was fed at a rate of 200 cc/min. and at a temperature of 130F. A mixture of 40 percent MEK and 60 percent MIBK was used as the solvent in the beginning. The MIBK concentration was increased in the lower tower stages to insure miscibility at the filtration temperature, that is, to reach an overall ratio of 25 percent MEK and percent MIBK at 15F. The solvent wa prechilled to a temperature between about +20 and about -20F. and introduced through the parallel inlets to give equivalent temperature drop per stage. The total solvent feed rate was 660 cc/min. The precipitated wax was removed from the oil-solvent solution by filtration at +l5F. The oil was recovered from the solution by distillation.

FIG. 3 shows that as the temperature at which the solvent is introduced into the chilling tower decreases the oil content of the wax decreases and that the best results are obtained below about 0F. for this solvent mixture.

What is claimed is:

l. A process for separating a wax-oil mixture comprising introducing said mixture, at a temperature about its cloud point, into a staged chilling zone, said mixture containing wax dissolved therein, contacting said wax-oil mixture therein with a solvent, said solvent introduced into said chilling zone at conditions such that said solvent is immiscible with said wax-oil mixture, and thereafter maintaining conditions in said chilling zone such that said solvent and said wax-oil mixture are miscible, maintaining a high degree of agitation in at least a portion of the stages containing said solvent and said wax-oil mixture so as to effect substantially instantaneous mixing of said solvent and said waxoil mixture, and cooling the wax-oil mixture as it progresses through said chilling zone thereby precipitating at least a portion of said wax from said waxoil mixture under conditions of said high degree of agitation.

2. The process of claim 1 wherein said solvent comprises a C C ketone or mixtures thereof.

3. The process of claim 2 wherein said solvent comprises 40 percent methylethylketone and 60 percent methylisobutylketone (by volume).

4. The process of claim 1 wherein said solvent comprises propylene-acetone.

5. The process of claim 1 wherein said solvent comprises methylethylketone and toluene.

6. The process of claim 1 wherein said solvent is introduced into the chilling zone at a temperature below about F.

7. A process for a waxy lubricating oil stock comprising introducing said waxy oil into a chilling zone divided into a plurality of stages, introducing a dewaxing solvent into at least a portion of said stages at conditions such that said solvent and said waxy oil are immiscible, and therafter maintaining conditions in said chilling zone such that said solvent and said oli are miscible, while maintaining a high degres of agitation in at least a portion of the stages containing said solvent and said waxy oil so as to effect substantially instantaneous mixing of said solvent and said waxy oil, cooling the waxy oil as it progresses through said chilling zone thereby precipitating at least a portion of said wax from said oil under conditions of said high degree of agitation, withdrawing a miscible oilsolvent mixture containing precipitated wax from said chilling zone and separating precipitated wax from said mixture at conditions such that said oil and said solvent are miscible.

oil stock from stage to stage of said chilling zone, injecting a cold dewaxing solvent into a plurality of said stages at a temperature and composition such that said solvent and said waxy oil are immiscible, and thereafter maintaining the temperature in said chilling zone such that said solvent and said waxy oil are miscible, maintaining a high degree of agitation in a plurality of the solvent/waxy oil-containing stages so as to effect substantially instantaneous mixing of said solvent and said waxy oil while cooling the waxy oil as it progresses through the chilling zone thereby precipitating a substantial portion of the wax from said oli stock, under conditions of said high degree of agitation, withdrawing a miscible oil-solvent mixture containing precipitated wax from said chilling zone, additionally cooling said miscible oil-solvent mixture thereby precipitating a further portion of the wax and separating the precipitated wax from said miscible oil-solvent mixture.

12. The process of claim 11 wherein said solvent comprises propylene and acetone.

13. The process of claim 11 wherein said solvent compses methylethylketone and methylisobutylketone.

14. A process for dewaxing a waxy lubricating oil fraction comprising introducing said fraction into a tower divided into a plurality of stages, passing said oil stock from stage to stage of said tower, injecting a cold dewaxing solvent into each of said stages at a temperature and composition such that said solvent and said oil are immiscible at the injection point, thereafter maintaining the temperature in said chilling zone and the solvent concentration in said oil stock such that said solvent and said oil stock are miscible, maintaining a high degree of agitation in each of said stages so as to effect substantially instantaneous mixing of said solvent and said oil, while cooling the waxy oil fraction as it progresses through said tower thereby precipitating at least a portion of the wax from said oil, under conditions of said high degree of agitation, withdrawing a miscible oil-solvent mixture comprising precipitated wax from said tower and separating the precipitated wax from said miscible oil-solvent mixture.

15. The process of claim 14 wherein said mixture is cooled at a rate ranging from about 2 to about 10F. per minute.

16. The process of claim 14 wherein said solvent comprises propylene-acetone.

17. The process of claim 14 wherein said solvent comprises methylethylketone and methylisobutylketone. 

2. The process of claim 1 wherein said solvent comprises a C3C6 ketone or mixtures thereof.
 3. The process of claim 2 wherein said solvent comprises 40 percent methylethylketone and 60 percent methylisobutylketone (by volume).
 4. The process of claim 1 wherein said solvent comprises propylene-acetone.
 5. The process of claim 1 wherein said solvent comprises methylethylketone and toluene.
 6. The process of claim 1 wherein said solvent is introduced into the chilling zone at a temperature below about 0*F.
 7. A process for a waxy lubricating oil stock comprising introducing said waxy oil into a chilling zone divided into a plurality of stages, introducing a dewaxing solvent into at least a portion of said stages at conditions such that said solvent and said waxy oil are immiscible, and therafter maintaining conditions in said chilling zone such that said solvent and said oli are miscible, while maintaining a high degres of agitation in at least a portion of the stages containing said solvent and said waxy oil so as to effect substantially instantaneous mixing of said solvent and said waxy oil, cooling the waxy oil as it progresses through said chilling zone thereby precipitating at least a portion of said wax from said oil under conditions of said high degree of agitation, withdrawing a miscible oilsolvent mixture containing precipitated wax from said chilling zone and separating precipitated wax from said mixture at conditions such that said oil and said solvent are miscible.
 8. The process of claim 7 wherein said solvent comprises propylene-acetone.
 9. ThE process of claim 7 wherein said solvent comprises a C3C6 ketone or mixtures thereof.
 10. The process of claim 9 wherein said solvent comprises 40 percent methylethylketone and 60 percent methylethylbutylketone (by volume).
 11. A process for dewaxing a waxy lubricating oil stock comprising introducing said waxy oil into a chilling zone divided into a plurality of stages, passing said oil stock from stage to stage of said chilling zone, injecting a cold dewaxing solvent into a plurality of said stages at a temperature and composition such that said solvent and said waxy oil are immiscible, and thereafter maintaining the temperature in said chilling zone such that said solvent and said waxy oil are miscible, maintaining a high degree of agitation in a plurality of the solvent/waxy oil-containing stages so as to effect substantially instantaneous mixing of said solvent and said waxy oil while cooling the waxy oil as it progresses through the chilling zone thereby precipitating a substantial portion of the wax from said oli stock, under conditions of said high degree of agitation, withdrawing a miscible oil-solvent mixture containing precipitated wax from said chilling zone, additionally cooling said miscible oil-solvent mixture thereby precipitating a further portion of the wax and separating the precipitated wax from said miscible oil-solvent mixture.
 12. The process of claim 11 wherein said solvent comprises propylene and acetone.
 13. The process of claim 11 wherein said solvent compses methylethylketone and methylisobutylketone.
 14. A process for dewaxing a waxy lubricating oil fraction comprising introducing said fraction into a tower divided into a plurality of stages, passing said oil stock from stage to stage of said tower, injecting a cold dewaxing solvent into each of said stages at a temperature and composition such that said solvent and said oil are immiscible at the injection point, thereafter maintaining the temperature in said chilling zone and the solvent concentration in said oil stock such that said solvent and said oil stock are miscible, maintaining a high degree of agitation in each of said stages so as to effect substantially instantaneous mixing of said solvent and said oil, while cooling the waxy oil fraction as it progresses through said tower thereby precipitating at least a portion of the wax from said oil, under conditions of said high degree of agitation, withdrawing a miscible oil-solvent mixture comprising precipitated wax from said tower and separating the precipitated wax from said miscible oil-solvent mixture.
 15. The process of claim 14 wherein said mixture is cooled at a rate ranging from about 2* to about 10*F. per minute.
 16. The process of claim 14 wherein said solvent comprises propylene-acetone.
 17. The process of claim 14 wherein said solvent comprises methylethylketone and methylisobutylketone. 